It is known in the art relating to automotive vehicles that engine durability is directly related to the lubricating ability of the engine crankcase oil, and that its lubricating ability becomes degraded with engine operation and time. Thus, most engine and vehicle manufacturers provide their customers with oil change maintenance schedules for the point at which the engine oil should be changed. These oil change maintenance schedules are only guidelines and, depending on the engine operating conditions, the required oil change interval may be less than 3,000 miles or greater than 10,000 miles. To provide a more accurate prediction of the point at which the oil should be changed, an estimate of the extent of oil degradation and the need for an oil change for a given vehicle can be determined by electronically monitoring certain key engine operating parameters in the course of vehicle operation between oil changes. When it is determined that an oil change is required, the operator is so informed by an instrument panel indicator.
It has been shown that a direct and accurate indication of engine oil degradation can be determined by assessing two effects: (1) chemical changes that occur as a consequence of exposure of the engine oil to high or low temperatures, without regard to engine loading or other operating conditions which may only be indirectly related to oil temperature, and (2) formation of contaminants such as soot and acids that typically are produced at high load and high temperature.
Excessive degradation of the engine oil occurs at its temperature extremes. At low engine oil temperatures, typically during start-up, fuel and water can accumulate in the engine oil. Fuel and fuel reaction products can also enter the engine oil and cause a decrease in oil viscosity. This low temperature effect can occur completely independent of the load.
At high engine oil temperatures, antioxidants in the oil can become inactivated, and thus a major additive that provides chemical stability to the engine oil is no longer as effective as it was initially. As a consequence, the oil becomes more viscous and acidic due to oxidation and nitration. In addition, insoluble materials may be deposited on the engine surfaces as a varnish or sludge.
Another type of oil degradation occurs as a function of the rate at which fuel is injected into the cylinders of the diesel engine. Contaminants such as soot and acids form during incomplete combustion, typically at high temperatures and at high loads. Soot and acids that have entered the engine oil reduce the ability of the oil to prevent corrosion and increase soot-related wear. Therefore, it is desirable to provide a monitoring system that determines the need to change the engine oil based on both the degradation of the engine oil due to high and low temperature effects and the degradation due to contamination from load-related soot and acid effects.